I've just finished a project with 12 daisy-chained TLC's. I'll put the project online next week. In my case I had each TLC on a separate board. The 12 boards were connected with 11x 20 cm ribbon cable. This gave me some problems with the signal after the 10th board. Because the boards go in a circle, instead of letting the signal travel from the source to the 12th (and back) I was able to split it so I had 0 - 5 and 6 - 11 for all the signals except for SOUT - SIN. That worked ok for me. But I don't think you'll have this problem when you put them all on one board.

When your plan is to control the 6 arduinos through usb than that shouldn't be a problem. But make sure that the configured baudrate is sufficient for your application. But that all depends on the resolution mode (256 (8bit) or 4096 (12bit)) and desired refresh rate.

I'm having some annoying probs here. I've connected the 5940 to my arduino NANO according to the schematic, triple-checked everything, but the leds still won't light. The problem seems to be voltage on the outputs. My multimeter reads about 0.2 V when i'm pumping 4000 on the channels. When alternating (slow blinking) the result is the expected (0V, 0.2V, 0V, 0.2V etc).My usb supply voltage is 4.5V, could that have anything to do with it?